Solenoid-controlled valves, or solenoid valves, are used in a variety of flow control applications to control the flow of fluids, including liquids, gases, and the like. In such applications, a control signal is typically provided from an external source that indicates when a valve is to be opened and closed in order to control the flow of fluids. A driver circuit receives the control signal and actuates the valve by supplying electrical power to the solenoid coil. The driver circuit typically includes a controller that is programed to apply the electrical power to the solenoid coil to energize the coil, which causes the valve to open (for a valve that is normally closed). After a specified amount of time, the controller removes the electrical power from the solenoid coil to deenergize the coil, which causes the valve to close.
In some solenoid valves, the controller does not completely remove electrical power when the valve is to be closed. In these solenoid valves, the controller allows a small supervisory or leakage current to flow instead of removing all electrical power completely. The controller uses the supervisory or leakage current as a wiring integrity monitoring signal to monitor and ensure the integrity of the wiring to the valve. While this scheme has many benefits, a potential drawback is the small supervisory or leakage current may inadvertently cause the solenoid coil to remain energized when it is supposed to be deenergized (i.e., “drop out”), which may prevent the valve from properly closing.
One way to ensure dropout of the solenoid coil is disclosed in commonly-assigned U.S. Pat. No. 8,925,566, entitled “System and Method of Assuring Drop Out of a Solenoid Valve,” which is incorporated herein by reference. This patent describes monitoring a control/actuation signal and, in a preferred embodiment, diverting all of that control/actuation signal to the solenoid coil if the signal is above a predetermined value (thereby energizing the solenoid coil), and diverting all of the control/actuation signal to a resistive load to be used as a wiring integrity signal otherwise (thereby ensuring the solenoid coil is deenergized).
The above solution, while appropriate for many solenoid valve applications, is less suitable for solenoid valves that use “peak-and-hold” drivers. The term “peak-and-hold”generally refers to a control scheme in which a large amount of electrical power (i.e., peak power) is initially applied to the solenoid coil to create a large initial pull-in force in order to move the valve mechanism to open the valve. Once the valve mechanism is moved, the large initial pull-in force is no longer needed and a smaller hold-in force requiring a smaller amount of electrical power (i.e., hold power) may be used to hold the valve open. Such a peak-and-hold scheme results in more efficient use of power and reduced heating of the solenoid coil.
Because the controller in a peak-and-hold driver applies two levels of power to the solenoid coil, a problem may arise if the controller allows a supervisory or leakage current to flow. Specifically, a condition may occur in peak-and-hold drivers where, as a result of the supervisory or leakage current, the controller may prematurely try to energize the solenoid coil during a dropout. This may result in current flowing in the windings of the solenoid coil that is large enough to not allow the solenoid coil to release the valve mechanism and properly close the valve.
Accordingly, a need exists for a way to assure dropout of a solenoid coil in a solenoid valve controlled by a peak-and-hold driver in which the controller allows a supervisory or leakage current to flow.